This invention relates to a picture signal encoding device for use in encoding an original picture signal with information compression. This invention relates also to a picture signal decoding device for use as a counterpart of the picture signal encoding device.
It will be assumed for the time being that the original picture signal represents a great number of original pictures in succession. Such an original picture signal comprises signal portions or sections representative of the respective original pictures. Merely by way of example, each original picture may represent a fingerprint. Typically, the information compression is achieved either by transform encoding or predictive encoding. The transform encoding is discussed in detail in a book written by William K. Pratt under the title of "Digital Image Processing" and published 1987 by John Wiley and Sons, pages 667 to 699, Section 23.2 entitled "Transform Coding". The predictive encoding is discussed in detail in the book of Pratt, pages 637 to 657, Section 22.5 entitled "Predictive Coding".
The following facts will later be described a little more in detail. When resorted to the transform encoding, the picture signal encoding device includes a dividing circuit for dividing each signal portion or section into signal blocks representative of block areas which are contiguous to one another in each original picture. Each block area is square in outline and consists of a plurality of picture elements, N.times.N in number, where N represents a predetermined natural number. The block areas therefore have a common size. The encoding device further includes a picture encoder for encoding the signal blocks into an encoded picture signal with a predetermined encoding characteristic.
Such a transform encoding device comprises a transformer or transform carrying out circuit. The transformer subjects each signal block to a linear transform to produce a transform coefficient signal representative of transform coefficients, N.times.N in number. With reference to a selection threshold value, a coefficient selector selects selected coefficients from the transform coefficients. A signal representative of the selected coefficients is encoded into the encoded picture signal. The selection threshold value is used in specifying at least partly the predetermined encoding characteristic. In a counterpart picture signal decoding device, the encoded picture signal is decoded into a reproduced picture signal representative of reproduced pictures which correspond to the respective original pictures.
In a conventional transform encoding device, the selection threshold value is predetermined and is invariable throughout the signal blocks. This adversely affects the reproduced picture signal and consequently the reproduced pictures. An improved transform encoding device is disclosed in an article contributed by Wein-Hsiung Chen et al to IEEE Transactions on Communications, Volume COM--25, No. 11 (Nov. 1977), pages 1285 to 1293, under the title of "Adaptive Coding of Monochrome and Color Images". According to Chen el al, the signal portions are classified into four classes in compliance with magnitudes of alternating-current energy. Different selection threshold values are predetermined for the respective classes. It appears, however, that use of the alternating-current energy magnitudes does not much improve the reproduced pictures.
In a predictive encoding device, no dividing circuit is used. Instead, attention is directed in the respective signal portions or sections to signal elements which represent the respective picture elements of the individual original pictures. On encoding the original picture signal into the encoded picture signal, the signal elements are successively encoded by using a prediction formula which is primarily defined by a plurality of prediction coefficients.
In a conventional predictive encoding device, the prediction coefficients are predetermined and are invariable throughout the signal portions. This adversely affects the reproduced picture signal and hence the reproduced pictures.